Gas turbine engines (GTE) produce power by extracting energy from a flow of hot gas produced by combustion of fuel in a stream of compressed air. In general, turbine engines have an upstream air compressor coupled to a downstream turbine with a combustion chamber (“combustor”) in between. Energy is released when a mixture of compressed air and fuel is burned in the combustor. The resulting hot gases are directed over blades of the turbine to spin the turbine and produce mechanical power. In a typical turbine engine, one or more fuel injectors direct a liquid or gaseous hydrocarbon fuel into the combustor for combustion. In some turbine engines, the fuel injectors are adapted to direct both a liquid fuel and gaseous fuel to the combustor (called dual fuel injectors). Depending upon availability, either a liquid fuel or a gaseous fuel may be directed to the combustor through these fuel injectors. In addition to producing power, combustion of hydrocarbon fuels in the combustor produces undesirable exhaust constituents such as NOx. It is desirable to reduce the emission of these undesirable constituents from GTEs. The formation of NOx in the combustor increases exponentially with the temperature of the flame in the combustor. Thus, a modest reduction in flame temperature can significantly reduce the emission of NOx from a GTE.
One technique used to reduce the emission of NOx from GTEs is to premix the fuel and air in the fuel injector to provide a lean fuel-air mixture to the combustor. This lean fuel-air mixture burns to produce a flame with a relatively low temperature and, thus, reduce NOx formation. However, a lean premixed fuel-air mixture may not be appropriate for all fuels. Fuels, such as synthesis gas (or any other fuel whose fundamental reaction rates, as indicated by the “laminar flame speed” SL, are very high) contain hydrogen, and may be prone to a phenomenon in which the flame front moves upstream against the flow of the air-fuel mixture, to cause an undesirable condition known as flashback. Other gaseous fuels and many liquid fuels are prone to a phenomenon known as autoignition. Autoignition is a phenomenon related to the chemical properties of the fuel whereby, when a fuel is mixed with an oxidizer, the oxidation reaction begins without the influence of an external source of energy such an electrical spark or another flame. Autoignition properties are well known for many fuels and are related to the pressure and temperature of the fuel and oxidizer mixture, and the time at which the mixture has been subject to those conditions. Lean Direct injection (LDI) of fuel into the combustor can be used to avoid flashback and autoignition. In an LDI system, fuel is directly injected into an air stream in a combustor and ignited in the combustor. However, if the fuel and air are not well mixed before combustion occurs, regions with higher fuel content may burn hotter and generate more NOx.
U.S. Pat. No. 7,536,862 B2 to Held et al. (the '862 patent) describes a fuel nozzle for a gas turbine engine in which fuel is injected from the nozzle tip into the combustor through a primary and a secondary opening. In the '862 patent, steam is injected alongside the fuel to decrease the temperature of the flame in the combustor, and thereby reduce NOx production. While the nozzle of the '862 patent may directly inject the fuel into the combustor and reduce NOx production, it may have limitations. For instance, injection of steam may detrimentally affect the efficiency of the gas turbine engine. Further, the fuel nozzle of the '862 patent is configured to inject only one type of fuel into the combustor and therefore may not be applicable to dual fuel injectors where two different fuels are supplied through the fuel injector. The method of operating the gas turbine engine of the current application may overcome these or other limitations in existing technology.